I've not had this happen to me, but my sister went through the same thing. Kept not having periods, kept getting negative results in urine tests so she went to the doctor. Blood tests came back negative. She started getting fatter, and she's a very slim girl so everyone took notice "Hey Sara, you're getting kinda chubby sweetheart" and then by 18 weeks the doctor finally said "You're pregnant!" Even at that it took an ultrasound to give us the shocking result that she was already that far along.
I personally get cysts here and there, mine tend to be very small though and pop on their own. They really aren't dangerous unless they are large, and occasionally they need to be surgically removed. You really should have some pain associated with it though. I would wait another week, see if your period doesn't start, your baby is only 9 months old and its possible that you are having hormone fluctuations yet.